This post deals with the retro Chevrolet HHR (model years 2006-2011). A Wikipedia entry provides some background information including that its styling can be attributed to Bryan Nesbitt, who also had a hand in styling the Chrysler PT Cruiser.
As best I can tell, the PT Cruiser, a sales success with more than one million sold, inspired General Motors to launch the HHR. However, the HHR was only a moderate marketplace success with sales less than half those of the PT Cruiser.
Styling was inspired by "Advance Design" Chevrolet trucks from the late 1940s and early 50s, though the HHR was in no way a copy.